K-5 with Astrotracer (focal lengths & ISO)?

johnha · 06-16-2018, 06:00 AM

Hi All,

I've got an O-GPS1 on it's way for my K-5 and looking for tips on which focal lengths and ISOs to use. I understand the trade-off between focal length & exposure duration, but I'm not sure if wide-angles or teles are better or whether pumping the ISO up helps or not. I appreciate that stacking multiple images gives better results, but this will be further down the line.

The widest prime I have is a K24mm, the widest zooms are the 17mm end of the DA17-70/4 and 20mm end of the FA20-35/4, longer lenses I could consider are the K135/2.5 and SMC-Tak 300/4. Is there a sweet spot focal length wise (given the constraints of composition) or any that are best avoided?

ISO wise, is there any benefit to cranking it up or am I better keeping it low?

John.

MaineNative · 06-16-2018, 06:53 AM

I have a K-5 with the O-GPS1 and a K-1. I am usually trying to shoot the Milky Way or landscapes, so I am usually going for something wider than 17 mm. I have the Samyang 14 mm/2.8 as my Astro lens. It is a lot easier if you have a lens showing focus distance markings so you know your focus is at infinity by looking. Some cheaper DA lenses don’t have the distance scale.

If you are going for deep space objects, you want the longer lenses but your Astrotracer time will be shorter.

For ISO, I have had good luck shooting 14 mm @ f3.5, ISO 800-1600 for 60-90 seconds. If I go much higher on ISO, I get a lot of noise and then for me Photoshop/Noiseware has a tough time figuring out what is noise and what is a faint star. If I start to go longer on Astrotracer time, I start getting small star trails or “elongated” stars. I have not played with stacking yet. If I stay too low on ISO I get fewer stars.

Another consideration is if you are shooting meteors, you need a faster ISO like 1600. The meteors are only there for a couple of seconds, but you can use Astrotracer to get 30-60 seconds to catch a meteor or two during meteor showers.

**microlight** · 06-16-2018, 07:15 AM

I did some Astro of the Milky Way a couple of nights ago with the K-3II and it worked pretty well. I used the 10mm end of my DA10-17 fisheye to get a big sky and took a few at ISO400 and 1600 at f4.5, with the shutter open for 180 seconds, and at this length of time there were no trails (apart from a faint diagonal line that was probably a satellite orbiting). This was followed by a shot with the Astrotracer off in order to get a sharp foreground for blending.

I read a while back that the Astrotracer (possibly the O-GPS1 too) only works up to an angle from the horizontal up to 30 degrees which seems to be true since I took a couple in New Zealand a few months back that showed trails when I pointed the camera up too far. So an ultrawide lens would assist.

DrawsACircle · 06-16-2018, 08:36 AM

This is 18x50sec ISO800 of the Whirlpool Galaxy with a M200mm on my K-70/O-GPS1.
It’s at a 70-80 degree angle, 50sec is pretty good with a 200mm lens, not something I expect everytime I use the Astrotracer program. I have my own rule of thumb, exposure time = (300/focal length)*10 which seems to work.

Last edited by DrawsACircle; 06-16-2018 at 10:22 AM.

**interested_observer** · 06-16-2018, 10:33 AM

There is an excellent discussion on astro lenses here...

Night photo/Astrotracer best lens advice - PentaxForums.com

As others have posted it all depends on what you want to do - Milky Way over landscapes - wider, deep space objects - more telephoto. Another favorite is wide star fields where something like 85mm to 135mm works well.

Faster lenses work better - f2.8 or faster, but read in the above link about larger physical aperture being better.

In terms of ISO, the K5 is ISO invariant which means you can pretty much use any ISO value and not be increasing noise.

Having said that, there is a reason why you want to maintain a low ISO and that is dynamic range and color. As you increase ISO you are loosing dynamic range. As dynamic range decreases, you will be loosing your ability to capture star color - essentially getting a lot of white stars.

So in a nutshell .....

If you use too wide of a wide angle lens / ultra wide angle lens - your physical aperture will decrease thereby limiting the amount of star light you will be capturing. The lenses that usually work the best (Milky Way) are Samyang 16/f2, Sigma 18-35/f1.8 and the Rokinon 24/f1.4. These also exhibit no coma. With these somewhat not as wide lenses, you tend to start to stitch scenes.

ISO 800-1600 seems to be a happy medium, where you get enough light/amplification and maintain sufficient dynamic range to get some reasonable colors. You can shoot at 800 to capture the dynamic range and then boost the exposure in post processing

Check out the Astrophotography group here on the forum. There is wonderful work down there in the last several years, along with some excellent guidance in terms of post processing work flow.

Astrophotography - PentaxForums.com

.... and let me toss this in just for effects. Overall, especially in post processing, I was starting to just see some strange things occasionally happening in post processing. Things would be ok, and then suddenly the entire image would just fall apart - with one very minor adjustment. This was even more apparent with my astro images. I picked up this little laptop for travel that has a wonderful screen and I found out that it was the extremely crappy pos monitor that I had on my tower system that was giving me the absolutely crappy post processing results. So, I've been using this little laptop until I figure out exactly what monitor I want to pick up.

johnha · 06-16-2018, 04:38 PM

Thanks for the info and links (interested_observer there are loads of very interesting points there for me to look at). I have a 10-17 FE which I had discounted for some unknown reason. I'll start with something wide and ISO 400-800 ish and go from there.

DrawsACircle - that's a very good image of the Whirlpool.

I do a bit of astro-observing and fully expect sometimes a given setup will work, while at others it won't for random, un-explainable and un-fathomable reasons.

06-16-2018, 06:00 AM	#1
johnha Veteran Member Join Date: Apr 2012 Location: Lancashire, UK Photos: Albums Posts: 1,155	K-5 with Astrotracer (focal lengths & ISO)? Hi All, I've got an O-GPS1 on it's way for my K-5 and looking for tips on which focal lengths and ISOs to use. I understand the trade-off between focal length & exposure duration, but I'm not sure if wide-angles or teles are better or whether pumping the ISO up helps or not. I appreciate that stacking multiple images gives better results, but this will be further down the line. The widest prime I have is a K24mm, the widest zooms are the 17mm end of the DA17-70/4 and 20mm end of the FA20-35/4, longer lenses I could consider are the K135/2.5 and SMC-Tak 300/4. Is there a sweet spot focal length wise (given the constraints of composition) or any that are best avoided? ISO wise, is there any benefit to cranking it up or am I better keeping it low? John.

06-16-2018, 10:33 AM	#5
interested_observer Loyal Site Supporter Join Date: Jun 2009 Location: Tumbleweed, Arizona Photos: Gallery \| Albums Posts: 5,707	There is an excellent discussion on astro lenses here... Night photo/Astrotracer best lens advice - PentaxForums.com As others have posted it all depends on what you want to do - Milky Way over landscapes - wider, deep space objects - more telephoto. Another favorite is wide star fields where something like 85mm to 135mm works well. Faster lenses work better - f2.8 or faster, but read in the above link about larger physical aperture being better. In terms of ISO, the K5 is ISO invariant which means you can pretty much use any ISO value and not be increasing noise. ISO Invariance: What it is, and which cameras are ISO-less How to Find the Best ISO for Astrophotography: Dynamic Range and Noise Having said that, there is a reason why you want to maintain a low ISO and that is dynamic range and color. As you increase ISO you are loosing dynamic range. As dynamic range decreases, you will be loosing your ability to capture star color - essentially getting a lot of white stars. So in a nutshell ..... If you use too wide of a wide angle lens / ultra wide angle lens - your physical aperture will decrease thereby limiting the amount of star light you will be capturing. The lenses that usually work the best (Milky Way) are Samyang 16/f2, Sigma 18-35/f1.8 and the Rokinon 24/f1.4. These also exhibit no coma. With these somewhat not as wide lenses, you tend to start to stitch scenes. ISO 800-1600 seems to be a happy medium, where you get enough light/amplification and maintain sufficient dynamic range to get some reasonable colors. You can shoot at 800 to capture the dynamic range and then boost the exposure in post processing Check out the Astrophotography group here on the forum. There is wonderful work down there in the last several years, along with some excellent guidance in terms of post processing work flow. Astrophotography - PentaxForums.com .... and let me toss this in just for effects. Overall, especially in post processing, I was starting to just see some strange things occasionally happening in post processing. Things would be ok, and then suddenly the entire image would just fall apart - with one very minor adjustment. This was even more apparent with my astro images. I picked up this little laptop for travel that has a wonderful screen and I found out that it was the extremely crappy pos monitor that I had on my tower system that was giving me the absolutely crappy post processing results. So, I've been using this little laptop until I figure out exactly what monitor I want to pick up. Last edited by interested_observer; 06-16-2018 at 10:47 AM. Reason: grammer

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06-16-2018, 06:53 AM	#2
MaineNative Senior Member Join Date: Jul 2017 Photos: Gallery Posts: 280	I have a K-5 with the O-GPS1 and a K-1. I am usually trying to shoot the Milky Way or landscapes, so I am usually going for something wider than 17 mm. I have the Samyang 14 mm/2.8 as my Astro lens. It is a lot easier if you have a lens showing focus distance markings so you know your focus is at infinity by looking. Some cheaper DA lenses don’t have the distance scale. If you are going for deep space objects, you want the longer lenses but your Astrotracer time will be shorter. For ISO, I have had good luck shooting 14 mm @ f3.5, ISO 800-1600 for 60-90 seconds. If I go much higher on ISO, I get a lot of noise and then for me Photoshop/Noiseware has a tough time figuring out what is noise and what is a faint star. If I start to go longer on Astrotracer time, I start getting small star trails or “elongated” stars. I have not played with stacking yet. If I stay too low on ISO I get fewer stars. Another consideration is if you are shooting meteors, you need a faster ISO like 1600. The meteors are only there for a couple of seconds, but you can use Astrotracer to get 30-60 seconds to catch a meteor or two during meteor showers.

06-16-2018, 07:15 AM	#3
microlight Site Supporter Join Date: Sep 2011 Location: Hampshire, UK Posts: 2,129	I did some Astro of the Milky Way a couple of nights ago with the K-3II and it worked pretty well. I used the 10mm end of my DA10-17 fisheye to get a big sky and took a few at ISO400 and 1600 at f4.5, with the shutter open for 180 seconds, and at this length of time there were no trails (apart from a faint diagonal line that was probably a satellite orbiting). This was followed by a shot with the Astrotracer off in order to get a sharp foreground for blending. I read a while back that the Astrotracer (possibly the O-GPS1 too) only works up to an angle from the horizontal up to 30 degrees which seems to be true since I took a couple in New Zealand a few months back that showed trails when I pointed the camera up too far. So an ultrawide lens would assist.

06-16-2018, 08:36 AM	#4
DrawsACircle Veteran Member Join Date: Dec 2016 Location: Ulfborg, Denmark Photos: Albums Posts: 434	This is 18x50sec ISO800 of the Whirlpool Galaxy with a M200mm on my K-70/O-GPS1. It’s at a 70-80 degree angle, 50sec is pretty good with a 200mm lens, not something I expect everytime I use the Astrotracer program. I have my own rule of thumb, exposure time = (300/focal length)10 which seems to work. Last edited by DrawsACircle; 06-16-2018 at 10:22 AM.*

06-16-2018, 04:38 PM	#6
johnha Veteran Member Join Date: Apr 2012 Location: Lancashire, UK Photos: Albums Posts: 1,155 Original Poster	Thanks for the info and links (interested_observer there are loads of very interesting points there for me to look at). I have a 10-17 FE which I had discounted for some unknown reason. I'll start with something wide and ISO 400-800 ish and go from there. DrawsACircle - that's a very good image of the Whirlpool. I do a bit of astro-observing and fully expect sometimes a given setup will work, while at others it won't for random, un-explainable and un-fathomable reasons.